Physical mapping of the transfer RNA genes on λh80d glyTsu+36

Abstract

The three Escherichia coli transfer RNA genes of the DNA of the transducing phage λ80 cI857S −t68d glyTsu + 36 tyrTthrT (abbreviated λh80T), which specify the structures of tRNA Gly 2( su + 36), tRNA Tyr 2 and tRNA Thr 3, have been mapped by hybridizing ferritin-labeled E. coli tRNA to heteroduplexes of λh80T DNA with the DNA of the parental phage (λh80c I857 S −t68) and examining the product in the electron microscope. The DNA of λh80T contains a piece of bacterial DNA of length 0·43 λ unit ‡ that replaces a piece of phage DNA of length 0·46 λ unit, proceeding left from B · P′ (the junction of bacterial DNA and phage DNA) (i.e. att 80). A cluster of three ferritin binding sites, and thus of tRNA genes, is seen at a position of 0·24 λ unit (1·1 × 10 4 nucleotides) to the left of B· P′. The three tRNA genes of the cluster are separated by the unequal spacings of 260 (±30) and 140 (± 30) nucleotides, proceeding left from B·P′. The specific map positions have been identified by hybridization competition between ferritin-labeled whole E. coli tRNA with unlabeled purified tRNA Tyr 2 and with unlabeled partially purified tRNA Gly 2. The central gene of the cluster is tRNA Tyr 2. The tRNA Gly 2gene is probably the one furthest from B·P′. Thus, the gene order and spacings, proceeding left from B·P′, are: tRNA Thr 3, 260 nucleotides, tRNA Try 2, 140 nucleotides, tRNA Gly 2.